Udawg and I were discussing this on vent sometime back. And basically what I mentioned is that when it comes to posting something, I generally look over the current page to see if there is a thread on it. I don't tend to look back over several pages of posts (and doing so could seem encombersome)...
He agreed and mentioned he only tended to look over the current page as well; and that if it was pushed that far back, there's a reason
Generally, it probably would be a good idea to see if there is a similar thread that was posted recently. But if it was like 300 threads back on page 5...I s'pose many wouldn't expect one to have searched the entire forum before making each post.
It's one of those things where it can also be good netequite to look over a thread before posting in it, and if it's reasonably short people probably tend to do so. But a thread that already has a thousand posts and has been active for well over a year, eh I don't know. Reading a few pages back might be reasonable in that case. I'm not sure what others would say about that though.